1. Field
This invention pertains generally to small modular pressurized water reactors and, more particularly, to a system for passively cooling a small modular reactor after the reactor has been tripped.
2. Related Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred into a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity with one another in a fuel assembly structure, through and over which the coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of the fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge on fissile material in adjacent fuel rods, contributing to the nuclear reaction and to the heat generated by the core.
Movable control rods are dispersed throughout the nuclear core to enable control of the overall rate of the fission reaction, by absorbing a portion of the neutrons, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to fission in an adjacent fuel rod; and retracting the control rods reduces the extent of neutron absorption and increases the rate of a nuclear reaction and the power output of the core.
FIG. 1 shows a simplified conventional nuclear reactor primary system, including a generally cylindrical pressure vessel 10 having a closure head 12 enclosing a nuclear core 14 that supports the fuel rods containing the fissile material. A liquid coolant, such as water or borated water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18 typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown) such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16 completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
Commercial power plants employing this design are typically on the order of 1,100 megawatts or more. More recently, Westinghouse Electric Company LLC has proposed a small modular reactor in the 200 megawatt class. The small modular reactor is an integral pressurized water reactor with all primary loop components located inside the reactor vessel. The reactor vessel is surrounded by a compact, high pressure containment. Due to both the limited spaced within the containment and the low cost requirement for integral pressurized light water reactors, the overall number of auxiliary systems needs to be minimized without compromising safety or functionality. For that reason, it is desirable to maintain most of the components in fluid communication with the primary loop of the reactor system within the compact, high pressure containment.
Typical conventional pressurized water reactor designs make use of active safety systems that rely on emergency AC power after an accident to power the pumps required to cool down the reactor and spent fuel pool. Advanced designs, like the AP1000®, offered by Westinghouse Electric Company LLC, make use of passive safety systems that only rely on natural circulation, boiling and condensation to remove the decay heat from the core and spent fuel pool. It is desirable to apply these passive safety system principals to a small modular reactor design and preferably simplify the design, while still maintaining the safety margins.
Accordingly, it is an object of this invention to provide a passive safety system for a small modular reactor that can identify the occurrence of a loss of coolant accident or a main steam line break and initiate a series of events within a pressurized containment of the small modular reactor to cool the reactor over an extended period of approximately five to seven days without outside intervention.
It is a further object of this invention to provide such a passive safety system that has a simplified design, which consolidates components over that previously employed in advanced larger reactor designs.